Simultaneous communication with multiple base stations

ABSTRACT

Various embodiments relate to simultaneously communicating with multiple base stations in an OFDM system. Each mobile terminal measures pilot signal strengths of transmissions from adjacent base stations. If the pilot signal strength for a base station exceeds the defined threshold, that base station is added to an active set list. Each mobile terminal notifies the base stations of their active set lists. By providing the set list to the base station controller and the servicing base station, the mobile terminal identifies the sole servicing base station or triggers a mode in which the mobile terminal simultaneously communicates with multiple base stations when the multiple base stations appear on the active set list. This mode uses a combination of scheduling and space-time coding to affect efficient and reliable (simultaneous) communication with the multiple base stations.

This application is a continuation of U.S. patent application Ser. No.12/983,636, filed Jan. 3, 2011, Ser. No. 12/983,636 is a continuation ofco-pending U.S. patent application Ser. No. 12/343,866, filed Dec. 24,2008, which is a continuation of U.S. patent application Ser. No.11/403,469, filed Apr. 13, 2006, which is a continuation of U.S. patentapplication Ser. No. 10/104,399, filed Mar. 22, 2002, now issued as U.S.Pat. No. 7,042,858, the disclosures of which are incorporated herein byreference in their entireties.

BACKGROUND

Wireless communication systems divide areas of coverage into cells, eachof which is served by a base station. A mobile terminal willcontinuously monitor the signal strengths of the servicing base stationfor the current cell as well as for adjacent cells. The mobile terminalwill send the signal strength information to the network. As the mobileterminal moves toward the edge of the current cell, the servicing basestation will determine that the mobile terminal's signal strength isdiminishing, while an adjacent base station will determine the signalstrength is increasing. The two base stations coordinate with each otherthrough the network, and when the signal strength of the adjacent basestation surpasses that of the current base station, control of thecommunications is switched to the adjacent base station from the currentbase station. The switching of control from one base station to anotheris referred to as a handoff.

A hard handoff is a handoff that completely and instantaneouslytransitions from a first to a second base station. Hard handoffs haveproven problematic and often result in dropped calls. CDMA systemsincorporate a soft handoff, wherein when the mobile terminal moves froma first to a second cell, the handoff process happens in multiple steps.First, the mobile terminal recognizes the viability of the second basestation, and the network allows both the current and adjacent basestations to carry the call. As the mobile terminal move closer to thesecond base station and away from the first base station, the signalstrength from the first base station will eventually drop below a usefullevel. At this point, the mobile terminal will inform the network, whichwill instruct the first base station to drop the call and let the secondbase station continue servicing the call. Accordingly, a soft handoff ischaracterized by commencing communications with a new base stationbefore terminating communications with the old base station. Softhandoffs in CDMA systems have proven very reliable.

In the ever-continuing effort to increase data rates and capacity ofwireless networks, communication technologies evolve.Multiple-input-multiple-output (MIMO) orthogonal frequency divisionmultiplexing (OFDM) systems represent an encouraging solution for thenext generation high-speed data downlink access. A benefit of suchsystems is their high spectral efficiency wherein all of the allocatedspectrum can be used by all base stations. The systems are generallyconsidered to have a frequency reuse factor of one. Unfortunately, thesesystems generate strong co-channel interference, especially at cellborders. Basic frequency reuse-one planning will lead to very low datarates and a poor quality of service for mobile terminals at cellborders. Even though data repetition, re-transmission techniques, andfairness scheduling for data transmission can be employed, it isdifficult to equalize data rate distribution across the cell.Accordingly, high-speed data service is severely limited.

In other technologies, such as CDMA, soft handoffs are used to enhanceservice at cell borders. However, a straightforward extension of softhandoff techniques developed for CDMA systems is not applicable to theMIMO-OFDM systems, since the separation of the interference for the OFDMwaveform is virtually impossible. Because different spreading codemasking is not available in OFDM transmission, the destructiveinterferences between base stations transmitting the same signal cancause significant degradation of performance. Accordingly, there is aneed for an efficient soft handoff technique for OFDM systems as well asa need to increase data rates and reduce interference at cell borders.

SUMMARY

One or more embodiments relate to soft handoffs in an OFDM system. Indownlink communications, each mobile terminal constantly measures all ofthe possible pilot signal strengths of transmissions from adjacent basestations, identifies the strongest pilot signals, and compares themagainst a defined threshold. If the pilot signal strength for a basestation exceeds the defined threshold, that base station is added to anactive set list. Each mobile terminal will notify the base stations oftheir active set lists. If there is only one base station in the activeset list, that base station is singled out to service the mobileterminal. If there is more than one base station on the active set list,a soft handoff is enabled between those base stations. The soft handoffcondition will continue until only one base station is on the active setlist, wherein the lone base station will continue to serve the mobileterminal. The soft handoff can be initiated by the mobile terminal,which will report the active set list to the base station controller viathe servicing base station. The base station controller will alert thebase stations on the active set list of the soft handoff. Notably, thebase station controller can select a sub-set of the base stations fromthe active set list to establish the soft hand off. During soft handoff,all base stations on the active set list will facilitate communicationswith the mobile terminal as defined below. At times, the base stationcontroller keeps track of all of the active set lists for the respectivemobile terminals. The mobile terminals will keep track of theirindividual set lists.

Accordingly, by providing the set list to the base station controllerand the servicing base station, the mobile terminal identifies the soleservicing base station or triggers a soft handoff (SHO) mode whenmultiple base stations appear on the active set list. The SHO mode usesa combination of scheduling and STC coding to affect efficient andreliable handoffs. During a SHO mode, the base station controller eithermulticasts or non-multicasts data packets intended for the mobileterminal to each of the base stations on the active set list,Multicasting indicates that each data packet is sent to each basestation on the active set list for transmission to the mobile terminal.Non-multicasting indicates that data packets are divided intosub-packets in some manner and each sub-packet is sent to one of thebase stations on the active set list for transmission to the mobileterminal. Unlike multicasting, redundant information is not transmittedfrom each base station on the active set list.

In either multicasting or non-multicasting embodiments, the basestations in the active set can partition the time and frequencyresources of the OFDM signal. Accordingly, each base station transmitspart of the OFDM signal sub-band. In some embodiments, a boost intransmit power is associated with sub-bands being used. The basestations provide STC encoding of the transmitted data and the mobileterminals provide corresponding STC decoding to recover the transmitteddata. The STC coding may be either space-time-transmit diversity (STTD)or V-BLAST-type coding. STTD coding encodes data into multiple formatsand simultaneously transmits the multiple formats with spatial diversity(i.e. from antennas at different locations). V-BLAST t-type codingseparates data into different groups and separately encodes andsimultaneously transmits each group. Other coding will be recognized bythose skilled in the art. The mobile terminal will separately demodulateand decode the transmitted data from each base station, and then combinethe decoded data from each base station to recover the original data.

Prior OFDM handoffs were hard handoffs, and the servicing base stationhandled scheduling of data for transmission for any given mobileterminal autonomously. Since only one base station served a mobileterminal at any one time, there was no need to employ joint scheduling.In contrast, some embodiments employ joint scheduling for base stationson the active set list of a mobile terminal. As such, the base stationcontroller or like scheduling device is used to schedule data packetsfor transmission during the SHO mode for each mobile terminal. Althoughthe base station controller may provide all scheduling for associatedbase stations, at least one embodiment delegates scheduling of data formobile terminals that are not in the SHO mode to the servicing basestation. In this arrangement, a scheduler is employed at the basestation controller to assign data to a time slot for the base stationson the active set list. The base stations perform joint base stationspace-time coding. The time slots not assigned by the base stationcontroller scheduler are used for data of mobile terminals notparticipating in a soft handoff.

Those skilled in the art will appreciate the scope of the variousembodiments, and realize additional aspects thereof after reading thefollowing detailed description of various embodiments in associationwith the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of one or moreembodiments.

FIG. 1 is a block representation of a cellular communication system.

FIG. 2 is a block representation of a base station according to one ormore embodiments.

FIG. 3 is a block representation of a mobile terminal according to oneor more embodiments.

FIG. 4 is a logical breakdown of an OFDM transmitter architectureaccording to one or more embodiments.

FIG. 5 is a logical breakdown of an OFDM receiver architecture accordingto one or more embodiments.

FIG. 6 is a table illustrating an active set list table according to oneor more embodiments.

FIG. 7A is a table illustrating round robin scheduling.

FIG. 7B is a table illustrating flexible scheduling.

FIGS. 8A-8C are a flow diagram outlining an exemplary operation of oneor more embodiments.

FIG. 9 is a block representation of a cellular communication systemconstructed according to one or more embodiments.

FIG. 10 is a diagram of frequency sub-band usage according to theembodiment of FIG. 9.

FIG. 11 is a block representation of a cellular communication systemconstructed according to one or more embodiments.

FIG. 12 is a diagram of frequency sub-band usage according to the one ormore embodiments of FIG. 11.

FIG. 13 is a diagram illustrating a technique for boosting the powerassociated with pilot signals while minimizing co-channel interferenceaccording to one or more embodiments.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice various embodiments. Uponreading the following description in light of the accompanying drawingfigures, those skilled in the art will understand various concepts, andwill recognize applications of these concepts not particularly addressedherein. It should be understood that these concepts and applicationsfall within the scope of the disclosure and the accompanying claims.

With reference to FIG. 1, a base station controller (BSC) 10 controlswireless communications within multiple cells 12, which are served bycorresponding base stations (BS) 14. In general, each base station 14will facilitate communications with mobile terminals 16, which arewithin the cell 12 associated with the corresponding base station 14. Asa mobile terminal 16 moves from a first cell 12 to a second cell 12,communications with the mobile terminal 16 transition from one basestation 14 to another. The term “handoff” is generally used to refer totechniques for switching from one base station 14 to another during acommunication session with a mobile terminal 16. The base stations 14cooperate with the base station controller 10 to ensure that handoffsare properly orchestrated, and that data intended for the mobileterminal 16 is provided to the appropriate base station 14 currentlysupporting communications with the mobile terminal 16.

Handoffs are generally characterized as either hard or soft. Hardhandoffs refer to handoffs where the transition from one base station 14to another is characterized by the first base station 14 stoppingcommunications with the mobile terminal 16 at the precise time when thesecond base station 14 begins communications with the mobile terminal16. Unfortunately, hard handoffs are prone to dropping communications,and have proven to be sufficiently unreliable. Soft handoffs arecharacterized by multiple base stations 14 simultaneously communicatingwith a mobile terminal 16 during a handoff period. Typically, the sameinformation is transmitted to the mobile terminal 16 from different basestations 14, and the mobile terminal 16 attempts to receive signals fromboth base stations 14 until the base station 14 to which the mobileterminal 16 is transitioning is deemed capable of taking overcommunications with the mobile terminal 16.

In FIG. 1, a handoff area 18 is illustrated at the junction of threecells 12, wherein a mobile terminal 16 is at the edge of any one of thethree cells 12 and could potentially be supported by any of the basestations 14 within those cells 12. One or more embodiments provide amethod and architecture for facilitating soft handoff in an orthogonalfrequency division multiplexing (OFDM) wireless communicationenvironment.

A high level overview of the mobile terminals 16 and base stations 14 ofone or more embodiments is provided prior to delving into associatedstructural and functional details. With reference to FIG. 2, a basestation 14 configured according to at least one embodiment isillustrated. The base station 14 generally includes a control system 20,a baseband processor 22, transmit circuitry 24, receive circuitry 26,multiple antennas 28, and a network interface 30. The receive circuitry26 receives radio frequency signals bearing information from one or moreremote transmitters provided by mobile terminals 16 (illustrated in FIG.3). In some cases, a low noise amplifier and a filter (not shown)cooperate to amplify and remove broadband interference from the signalfor processing. Down-conversion and digitization circuitry (not shown)will then downconvert the filtered, received signal to an intermediateor baseband frequency signal, which is then digitized into one or moredigital streams.

The baseband processor 22 processes the digitized received signal toextract the information or data bits conveyed in the received signal.This processing typically comprises demodulation, decoding, and errorcorrection operations. As such, the baseband processor 22 is generallyimplemented in one or more digital signal processors (DSPs). Thereceived information is then sent across a wireless network via thenetwork interface 30 or transmitted to another mobile terminal 16serviced by the base station 14. The network interface 30 will typicallyinteract with the base station controller 10 and a circuit-switchednetwork forming a part of a wireless network, which may be coupled tothe public switched telephone network (PSTN).

On the transmit side, the baseband processor 22 receives digitized data,which may represent voice, data, or control information, from thenetwork interface 30 under the control of control system 20, whichencodes the data for transmission. The encoded data is output to thetransmit circuitry 24, where it is modulated by a carrier signal havinga desired transmit frequency or frequencies. A power amplifier (notshown) will amplify the modulated carrier signal to a level appropriatefor transmission, and deliver the modulated carrier signal to theantennas 28 through a matching network (not shown). Modulation andprocessing details are described in greater detail below.

With reference to FIG. 3, a mobile terminal 16 configured according toat least one embodiment is illustrated. Similarly to the base station14, the mobile terminal 16 will include a control system 32, a basebandprocessor 34, transmit circuitry 36, receive circuitry 38, multipleantennas 40, and user interface circuitry 42. The receive circuitry 38receives radio frequency signals bearing information from one or morebase stations 14. In some embodiments, a low noise amplifier and afilter (not shown) cooperate to amplify and remove broadbandinterference from the signal for processing. Down-conversion anddigitization circuitry (not shown) will then downconvert the filtered,received signal to an intermediate or baseband frequency signal, whichis then digitized into one or more digital streams.

The baseband processor 34 processes the digitized received signal toextract the information or data bits conveyed in the received signal.This processing typically comprises demodulation, decoding, and errorcorrection operations, as will be discussed on greater detail below. Thebaseband processor 34 is generally implemented in one or more digitalsignal processors (DSPs) and application specific integrated circuit(ASIC).

For transmission, the baseband processor 34 receives digitized data,which may represent voice, data, or control information, from thecontrol system 32, which it encodes for transmission. The encoded datais output to the transmit circuitry 36, where it is used by a modulatorto modulate a carrier signal that is at a desired transmit frequency orfrequencies. A power amplifier (not shown) will amplify the modulatedcarrier signal to a level appropriate for transmission, and deliver themodulated carrier signal to the antennas 40 through a matching network(not shown). Various modulation and processing techniques available tothose skilled in the art are applicable to one or more embodiments.

In OFDM modulation, the transmission band is divided into multiple,orthogonal carrier waves. Each carrier wave is modulated according tothe digital data to be transmitted. Because OFDM divides thetransmission band into multiple carriers, the bandwidth per carrierdecreases and the modulation time per carrier increases. Since themultiple carriers are transmitted in parallel, the transmission rate forthe digital data, or symbols, on any given carrier is lower than when asingle carrier is used.

OFDM modulation requires the performance of an Inverse Fast FourierTransform (IFFT) on the information to be transmitted. For demodulation,the performance of a Fast Fourier Transform (FFT) on the received signalis required to recover the transmitted information. In practice, theInverse Discrete Fourier Transform (IDFT) and Discrete Fourier Transform(DFT) are implemented using digital signal processing for modulation anddemodulation, respectively.

Accordingly, the characterizing feature of OFDM modulation is thatorthogonal carrier waves are generated for multiple bands within atransmission channel. The modulated signals are digital signals having arelatively low transmission rate and capable of staying within theirrespective bands. The individual carrier waves are not modulateddirectly by the digital signals. Instead, all carrier waves aremodulated at once by IFFT processing.

In at least one embodiment, OFDM is used at least for the downlinktransmission from the base stations 14 to the mobile terminals 16.Further, the base stations 14 are synchronized to a common clock via GPSsignaling and coordinate communications via the base station controller10. Each base station 14 is equipped with n transmit antennas 28, andeach mobile terminal 16 is equipped with m receive antennas 40. Notably,the respective antennas can be used for reception and transmission usingappropriate duplexers or switches and are so labeled only for clarity.

With reference to FIG. 4, a logical OFDM transmission architecture isprovided according to one embodiment. Initially, the base stationcontroller 10 sends data 44 to be transmitted to a mobile terminal 16 tothe base station 14. The data, which is a stream of bits, is scrambledin a manner reducing the peak-to-average power ratio associated with thedata using data scrambling logic 46. A cyclic redundancy check (CRC) forthe scrambled data is determined and appended to the scrambled datausing CRC logic 48. Next, channel coding is performed using channelencoder logic 50 to effectively add redundancy to the data to facilitaterecovery and error correction at the mobile terminal 16. The channelencoder logic 50 uses known Turbo encoding techniques in one embodiment.The encoded data is then processed by rate matching logic 52 tocompensate for the data expansion associated with encoding.

Bit interleaver logic 54 systematically reorders the bits in the encodeddata to minimize the loss of consecutive data bits is provided by. Theresultant data bits are systematically mapped into corresponding symbolsdepending on the chosen baseband modulation by mapping logic 56. Attimes, Quadrature Amplitude Modulation (QAM) or Quadrature Phase ShiftKey (QPSK) modulation can be used. The symbols may be systematicallyreordered to further bolster the immunity of the transmitted signal toperiodic data loss caused by frequency selective fading using symbolinterleaver logic 58.

At this point, groups of bits have been mapped into symbols representinglocations in an amplitude and phase constellation. Blocks of symbols arethen processed by space-time block code (STC) encoder logic 60, whichmodifies the symbols in a fashion making the transmitted signals moreresistant to interference and readily decoded at a mobile terminal 16.The STC encoder logic 60 will process the incoming symbols and provide noutputs corresponding to the number of transmit antennas 28 for the basestation 14. The control system 20 and/or baseband processor 22 willprovide a mapping control signal to control STC encoding. Further detailregarding the STC encoding is provided later in the description. At thispoint, assume the symbols for the n outputs are representative of thedata to be transmitted and capable of being recovered by the mobileterminal 16. See A. F. Naguib, N. Seshadri, and A. R. Calderbank,“Applications of space-time codes and interference suppression for highcapacity and high data rate wireless systems,” Thirty-Second AsilomarConference on Signals, Systems & Computers, Volume 2, pp. 1803-1810,1998, which is incorporated herein by reference in its entirety.

For the present example, assume the base station 14 has two antennas 28(n=2) and the STC encoder logic 60 provides two output streams ofsymbols. Accordingly, each of the symbol streams output by the STCencoder logic 60 is sent to a corresponding IFFT processor 62,illustrated separately for ease of understanding. Those skilled in theart will recognize that one or more processors may be used to providesuch digital signal processing alone or in combination with otherprocessing described herein. The IFFT processors 62 can operate on therespective symbols using IDFT or like processing to effect an inverseFourier Transform. The output of the IFFT processors 62 provides symbolsin the time domain. The time domain symbols are grouped into frames,which are associated with prefix and pilot headers by like insertionlogic 64. Each of the resultant signals is up-converted in the digitaldomain to an intermediate frequency and converted to an analog signalvia the corresponding digital up-conversion (DUC) and digital-to-analog(D/A) conversion circuitry 66. The resultant (analog) signals are thensimultaneously modulated at the desired RF frequency, amplified, andtransmitted via the RF circuitry 68 and antennas 28. Notably, thetransmitted data is preceded by pilot signals, which are known by theintended mobile terminal 16 and implemented by modulating the pilotheader and scattered pilot sub-carriers. The mobile terminal 16, whichis discussed in detail below, will use the scattered pilot signals forchannel estimation and interference suppression and the header foridentification of the base station 14.

Reference is now made to FIG. 5 to illustrate reception of thetransmitted signals by a mobile terminal 16. Upon arrival of thetransmitted signals at each of the antennas 40 of the mobile terminal16, the respective signals are demodulated and amplified bycorresponding RF circuitry 70. For the sake of conciseness and clarity,only one of the two receive paths is described and illustrated indetail. Analog-to-digital (ND) converter and down-conversion circuitry72 digitizes and downconverts the analog signal for digital processing.The resultant digitized signal may be used by automatic gain controlcircuitry (AGC) 74 to control the gain of the amplifiers in the RFcircuitry 70 based on the received signal level.

In one or more embodiments, each transmitted frame has a definedstructure having two identical headers. Framing acquisition is based onthe repetition of these identical headers. Initially, the digitizedsignal is provided to synchronization logic 76, which includes coarsesynchronization logic 78, which buffers several OFDM symbols andcalculates an auto-correlation between the two successive OFDM symbols.A resultant time index corresponding to the maximum of the correlationresult determines a fine synchronization search window, which is used bythe fine synchronization logic 80 to determine a precise framingstarting position based on the headers. The output of the finesynchronization logic 80 facilitates frame acquisition by the framealignment logic 84. Proper framing alignment is important so thatsubsequent FFT processing provides an accurate conversion from the timeto the frequency domain. The fine synchronization algorithm is based onthe correlation between the received pilot signals carried by theheaders and a local copy of the known pilot data. Once frame alignmentacquisition occurs, the prefix of the OFDM symbol is removed with prefixremoval logic 86 and a resultant samples are sent to frequency offsetand Doppler correction logic 88, which compensates for the systemfrequency offset caused by the unmatched local oscillators in thetransmitter and the receiver and Doppler effects imposed on thetransmitted signals. In some cases, the synchronization logic 76includes frequency offset, Doppler, and clock estimation logic, which isbased on the headers to help estimate such effects on the transmittedsignal and provide those estimations to the correction logic 88 toproperly process OFDM symbols.

At this point, the OFDM symbols in the time domain are ready forconversion to the frequency domain using the FFT processing logic 90.The results are frequency domain symbols, which are sent to processinglogic 92. The processing logic 92 extracts the scattered pilot signalusing scattered pilot extraction logic 94, determines a channel estimatebased on the extracted pilot signal using channel estimation logic 96,and provides channel responses for all sub-carriers using channelreconstruction logic 98. The frequency domain symbols and channelreconstruction information for each receive path are provided to an STCdecoder 100, which provides STC decoding on both received paths torecover the transmitted symbols. The channel reconstruction informationprovides the STC decoder 100 sufficient information to process therespective frequency domain symbols to remove the effects of thetransmission channel.

The recovered symbols are placed back in order using the symbolde-interleaver logic 102, which corresponds to the symbol interleaverlogic 58 of the transmitter. The de-interleaved symbols are thendemodulated or de-mapped to a corresponding bit stream using de-mappinglogic 104. The bits are then de-interleaved using bit de-interleaverlogic 106, which corresponds to the bit interleaver logic 54 of thetransmitter architecture. The de-interleaved bits are then processed byrate de-matching logic 108 and presented to channel decoder logic 110 torecover the initially scrambled data and the CRC checksum. Accordingly,CRC logic 112 removes the CRC checksum, checks the scrambled data intraditional fashion, and provides it to the de-scrambling logic 114 forde-scrambling using the known base station de-scrambling code to recoverthe originally transmitted data.

Since OFDM is a parallel transmission technology, the entire usefulbandwidth is divided into many-sub-carriers, which are modulatedindependently. A common synchronization channel, a pilot channel, and abroadcasting channel are multiplexed into the header of the OFDM symbolin the frequency domain based on the sub-carrier position. The commonsynchronization channel is used for initial acquisition for timingsynchronization, frequency and Doppler estimation, and initial channelestimation.

In one embodiment, 256 common synchronization sub-carriers are furtherdivided between the respective transmission paths wherein each path isassociated with 128 common synchronization sub-carriers, respectively. Acommon complex PN code of size 256, which is shared by both transmitpaths, is used to modulate the sub-carriers reserved for the commonsynchronization channels.

The pilot channel is used for synchronization, initial channelestimation, base station identification, and carrier-to-interferenceratio (CIR) measurements for cell (or base station) selection. In oneembodiment, 256 sub-carriers are reserved for dedicated pilots whereineach transmission path has 128 pilot sub-carriers. A unique complex PNcode with length 256 is assigned to each base station 14 and mapped tothese dedicated pilots. The orthogonality of the PN codes assigned tothe different base stations 14 provides for base station identificationand initial interference measurement.

In one embodiment, the frame structure has two identical header symbolsat the beginning of every 10 msec. frame. The framing acquisition isbased on the repeated headers. When turned on, the mobile terminal 16will start the time domain coarse synchronization processing. A runningbuffer is used to buffer several OFDM symbols, and then calculate theauto-correlation between two successful OFDM symbols. The coarsesynchronization position is the time index corresponding to the maximumoutput of the auto-correlations.

After framing acquisition, only the rough range of the location of thestarting position of the first header symbol is known. To perform OFDMmodulation in the frequency domain, the starting location of OFDM symbolmust be exact so the FFT can transfer the signals from the time domainto the frequency domain. Accordingly, the location of the first samplein the first header of the OFDM symbol is determined. Finesynchronization is based on the correlation between the pilot data inthe headers of the received signals and a local copy of the known pilotdata.

With regard to channel estimation, each sub-band, which is representedby a modulated sub-carrier, only covers a small fraction of the entirechannel bandwidth. The frequency response over each individual sub-bandis relatively flat, which makes coherent demodulation relatively easy.Since the transmission channel corrupts the transmitted signal inamplitude and phase, reliable channel knowledge is required to performcoherent detection. As noted, one embodiment uses a pilot signal forchannel parameter estimation to keep track of channel characteristicscaused by the movement of the mobile terminal 16. Accordingly, scatteredpilot signals are inserted in a regular pattern. The pilot signals areperiodically interpolated to obtain current channel information requiredfor STC decoding.

Based on the above, system access is characterized as follows.Initially, coarse synchronization correlation is performed based on thepreamble header in the time domain to determine a coarse synchronizationlocation. At the coarse synchronization location, a fine synchronizationsearch window is identified. An FFT is computed, and the system switchesto the common synchronization channel to perform fine synchronizationwithin the fine synchronization search window. Next, the strongestcorrelation peaks are identified, and the relevant time index are usedas the candidate timing synchronization positions. An FFT is computed ateach candidate timing synchronization position, and the system switchesto the pilot channel.

The PN sequences for all base stations 14 are correlated, andcorrelation peaks are selected to define an index corresponding to allcandidate timing synchronization positions. The CIRs for these basestations 14 are identified. The base station with highest CI R isselected as the serving base station, and the base stations 14 with CIRsgreater than a given threshold are also selected for the active setlist. If more than one base station 14 is on the active set list, softhandoff procedures are initiated. The FFT is then computed and the finesynchronization is provided using the PN code for each of the selectedbase station(s) 14.

During operation, the transmitter architecture of the mobile terminal 16will facilitate system access as follows. In general, downlinkcommunications from a base station 14 to a mobile terminal 16 areinitiated by the mobile terminal 16. Each mobile terminal 16 constantlymeasures all of the possible pilot signal strengths of transmissionsfrom adjacent base stations 14, identifies the strongest pilot signals,and compares them against a defined threshold. If the pilot signalstrength for a base station 14 exceeds the defined threshold, that basestation 14 is added to an active set list. Each mobile terminal 16 willnotify the base stations 14 of their active set lists. If there is onlyone base station 14 in the active set list, that base station 14 issingled out to service the mobile terminal 16. If there is more than onebase station 14 on the active set list, a soft handoff is enabledbetween those base stations 14. The soft handoff condition will continueuntil only one base station 14 is on the active set list, wherein thelone base station 14 will continue to serve the mobile terminal 16.During soft handoff, all base stations 14 on the active set list willfacilitate communications with the mobile terminal 16 as defined below.In some embodiments, the base station controller 10 keeps track of allof the active set lists for the respective mobile terminals 16. Themobile terminals 16 will keep track of their individual set lists.

Accordingly, by providing the set list to the base station controller 10and the servicing base station 14, the mobile terminal 16 identifies thesole servicing base station 14 or triggers a soft handoff (SHO) modewhen multiple base stations appear on the active set list. The SHO modeuses a combination of scheduling and STC coding to affect efficient andreliable handoffs. During a SHO mode, the base station controller 10either multicasts or non-multicasts data packets intended for the mobileterminal 16 to each of the base stations 14 on the active set list.Multicasting indicates that each data packet is sent to each basestation 14 on the active set list for transmission to the mobileterminal 16. Non-multicasting indicates that data packets are dividedinto sub-packets in some manner and each sub-packet is sent to one ofthe base stations 14 on the active set list for transmission to themobile terminal 16. Unlike multicasting, redundant information is nottransmitted from each base station 14 on the active set list.

In either multicasting or non-multicasting embodiments, the basestations 14 provide STC encoding of the transmitted data and the mobileterminals 16 provide corresponding STC decoding to recover thetransmitted data. The STC coding may be either space-time-transmitdiversity (STTD) or V-BLAST-type coding. STTD coding encodes data intomultiple formats and simultaneously transmits the multiple formats withspatial diversity (i.e. from antennas 28 at different locations).V-BLAST-type coding separates data into different groups and separatelyencodes and simultaneously transmits each group with spatial diversity.Other coding will be recognized by those skilled in the art. The mobileterminal 16 will separately demodulate and decode the transmitted datafrom each base station 14, and then combine the decoded data from eachbase station 14 to recover the original data.

The following illustrates an exemplary process for identifying basestations 14 to place in the active set list, scheduling of data at thebase stations 14, and STC coding for transmission of scheduled data fromthe base stations 14 to the mobile terminals 16.

For a multiple-input-multiple-output (MIMO) OFDM system as illustratedin FIG. 1, the pilot signal is embedded in the preamble of each framefor each base station 14. The mobile terminal 16 can identify each basestation 14 based on the pseudo-noise sequence of the pilot signal. Themobile terminal 16 is able to measure the carrier-to-interference ratio(CIR) based on the pilot signal for each adjacent base station 14. Basedon the strength of the pilot signal, the mobile terminal 16 candetermine the active set list. If more than one base station 14 is onthe active set list, the mobile terminal 16 will trigger SHO procedurethrough the uplink signaling with the base station 14, which willcommunicate the same to the base station controller 10.

With reference to FIG. 6, an exemplary active set list for acommunication environment is shown. Assume that a single base stationcontroller 10 controls the operation of nine base stations, BS 1 -BS 9 .Further assume that there are fifteen mobile terminals 16 identified asmobile terminals A-O within the communication environment, and that allof the mobile terminals (A-0) are in handoff areas from which servicemay be provided by two or three of the base stations BS 1 -BS 9 . Theshaded areas of the active set list tables identify the active set listsof base stations BS 1 -BS 9 for each of the mobile terminals A-O. In thepresent example, mobile terminals A, B, F, G, K, and L are involved in atwo-way SHO procedure wherein two of the base stations BS 1 -BS 9 are onthe corresponding mobile terminals' active set lists. Similarly, mobileterminals C, D, E, H, I, J, M, N, and O are in a three-way SHOprocedure, wherein three of the base stations BS 1 -BS 9 are on thecorresponding mobile terminals' active set lists. For example, theactive set list of mobile terminal B identifies base stations BS 3 andBS 5 and the active set list for mobile terminal H identifies basestations BS 1 , BS 6 , and BS 7. As noted, once these mobile terminalsA-O determine that there are multiple base stations BS 1 -BS 9 on theactive set list, the mobile terminal 16 will trigger a SHO procedurethrough uplink signaling with its currently servicing base station 14.The base station 14 will alert the base station controller 10, whichwill begin the SHO procedure.

Prior OFDM handoffs were hard handoffs, and the servicing base station14 handled scheduling of data for transmission for any given mobileterminal 16 autonomously. Since only one base station 14 served a mobileterminal 16 at any one time, there was no need to employ jointscheduling. In contrast, some embodiments employ joint scheduling forbase stations 14 on the active set list of a mobile terminal 16. Assuch, the base station controller 10 and not the serving base station 14is used to schedule data packets for transmission during the SHO modefor each mobile terminal 16. Although the base station controller 10 mayprovide all scheduling for associated base stations 14, some embodimentsdelegate scheduling of data for mobile terminals 16 that are not in theSHO mode to the servicing base station 14.

In order to minimize the complexity of the system, the base stationcontroller 10 classifies the active mobile terminals 16 into twocategories: (1) SHO and (2) non-SHO. For a non-SHO mobile terminal 16,each base station 14 will schedule packet transmissions independentlybased on the channel quality reported at that particular base station 14by the mobile terminal 16. For example, the scheduling may be based onmaximum CIR scheduling, round robin scheduling, or any other schedulingprovision known in the art. For a SHO mobile terminal 16, the basestation controller 10 may use a simple round robin scheduler and mayeither multicast or non-multicast the packets to the base stations 14 onthe active set list at a given time slot.

For multicast, each data packet is sent to each base station 14 on theactive set list for transmission to the mobile terminal 16. Fornon-multicast, data packets are divided into sub-packets in some mannerand each sub-packet is sent to one of the base stations 14 on the activeset list for transmission to the mobile terminal 16. In the latter case,there is no redundancy among the bases stations 14. Each base station 14sends a unique piece of the data being transmitted. When SHO-modescheduling is not required, the serving base stations 14 will scheduleand transmit data to mobile terminals 16 in the non-SHO mode. The roundrobin scheduling provided by the base station controller 10 for a mobileterminal 16 in SHO mode can be determined by the ratio of the number ofSHO-mode mobile terminals to the non-SHO-mode mobile terminals 16.Alternatively, the scheduling may be controlled to maximize capacity,minimize delay, etc. The packet transmission for a SHO mode can besignaled via fast downlink signaling.

An exemplary round robin scheduling technique for the base stationcontroller 10 is illustrated in FIG. 7A in light of the active set listinformation provided in FIG. 6. As depicted, communications between abase station 14 and a mobile terminal 16 are assigned to a given timeslot in a scheduling period. The base station controller 10 schedulescommunications for designated time slots for mobile terminals 16operating in a SHO mode and leaves the shaded time slots open fortraditional, non-SHO mode scheduling at the respective base stations 14.Accordingly, the base station controller 10 will schedule data to besent to each of the base stations 14 participating in a SHO mode with agiven mobile terminal 16 for a common time slot. For example, data to betransmitted to mobile terminal I is scheduled for time slot 1 for basestations BS 1 , BS 6 , and BS 7 . Data to be transmitted to mobileterminal C is scheduled for time slot 1 and sent to base stations BS 3 ,BS 4 , and BSS. Similarly, data to be transmitted to mobile terminal Ois also scheduled for time slot 1 and delivered to base stations BS 2 ,BSB, and BS 9 on its active set list. Thus, data to be transmitted to amobile terminal 16 in a SHO mode is scheduled for a common time slot foreach of the base stations 14 in the active set list. To minimize theprocessing required for round robin scheduling, the allocation of timeslots for the various mobile terminals 16 participating in the SHO modeare kept consistent from one scheduling period to the next until thereis a change in the active set list for one or more of the mobileterminals 16. As illustrated, the allocation of communications for themobile terminals 16 for time slot 1 and 13 are identical, and so on andso forth. Once the base stations 14 provide the multicasting ornon-multicasting of the SHO mode data, the base stations 14 can providescheduling during the shaded time slots for mobile terminals 16 that arenot operating in the SHO mode.

FIG. 7B illustrates an alternative scheduling arrangement, wherein thescheduling for SHO mode and non-SHO mode mobile terminals 16 is notrepeated from one scheduling period to another, but is recomputed andreassigned during each scheduling period. During time slot 1 , data tobe transmitted to mobile terminal I is sent to base stations BS 1 , BS 6, and BS 7 , wherein data to be transmitted to mobile terminal L is sentto base stations BS 2 and BS 9 . Base stations BS 3 , BS 4 , BSS, and BS8 are free to schedule data to non-SHO mode mobile terminals 16.Corresponding time slot 13 in the subsequent scheduling period does notparallel the allocations of time slot 1 . The base station controller 10will compute a different scheduling and slot allocation procedure forthe scheduling period, wherein mobile terminals J and O, which areoperating in the SHO mode, are scheduled to have data transmitted tobase stations BS 1 , BS 6 , and BS 7 , and base stations BS 2 , BS 8 ,and BS 9 , respectively. Those skilled in the art will recognize thenumerous ways to facilitate scheduling for SHO mode terminals via thebase station controller 10 while allocating time slots for the basestations 14 to provide scheduling for mobile terminals not operating ina SHO mode.

Regardless of scheduling techniques, each base station 14 on the activeset will perform the space-time coding at the same time during theassigned time slot. Accordingly, the mobile terminal 16 will receive theentire space-time coded data packet transmitted from the multiple basestations 14. The mobile terminal 16 will separately demodulate anddecode the transmitted data from each base station 14, and then combinethe decoded data from each base station 14 to recover the original data.

With reference to FIGS. 8A-8C, an exemplary flow of an active SHOprocess is described. Initially, a mobile terminal 16 will measure thepilot signal strength of each base station (step 200) and compute thecarrier-to-interference ratio (CIR) using equation 1 (step 202).CIR ₀ =C/(I ₁ +I ₂ +I ₃ + . . . +I _(n)),  Equation 1wherein C is a measure of the pilot signal strength of the servicingbase station 14 and I₁ through I_(N) are measures of the pilot signalstrengths for adjacent base stations 14 (BS 1 through BSN). If thecomputed CIR is greater than a threshold CIR (Th₀) (step 204), themobile terminal 16 will maintain the servicing base station 14 in theactive set list, and not add any of the adjacent base stations 14 to theactive set list. Thus, the mobile terminal 16 will receivecommunications only from the servicing base station 14 and will not bein a SHO mode (step 206). If the computed CIR is not greater than thethreshold CIR Th₀, the mobile terminal 16 will compute another CIR usingequation 2 (step 208).CIR ₁=(C+I ₁)/(I ₂ +I ₃ + . . . +I _(N)).  Equation 2

If CIR₁ is greater than the threshold CIR (step 210), the mobileterminal 16 will trigger a two-way SHO between the servicing basestation 14 and the adjacent base stations 14 from which I₁ was measured(step 212). If CIR₁ was not greater than the threshold CIR (step 210),then the mobile terminal 16 computes another CIR using equation 3 (step214).CIR ₃=(C+I ₁ +I ₂ +I ₃)/(I ₄ + . . . +I _(N)),  Equation 3If CIR₂ is greater than the threshold CIR (step 216), the mobileterminal 16 will trigger a three-way SHO mode with the servicing basestation 14 and the adjacent base stations 14 associated with I₁ andI₂(step 218). If CIR₂ is not greater than the threshold CIR (step 216),the mobile terminal 16 will compute a new CIR according to equation 4(step 220),CIR ₃=(C+I ₁ +I ₂ +I ₃)/(I ₄ + . . . +I _(N)),  Equation 4and the process will continue by adding an adjacent interferencecomponent from adjacent base stations 14 until a sufficient, combinedCIR exceeds the threshold CIR Th₀.

For the present example, assume that a two-way SHO procedure wastriggered wherein the flow moves to FIG. 8B. Once the mobile terminal 16achieves a CIR greater than the threshold CIR, it will send informationidentifying the base stations 14 on the active set list and thecalculated CIR to the serving base station 14 (step 222). The servingbase station 14 will report the active set list and the calculated CIRto the base station controller 10 (step 224). The base stationcontroller 10 grants the SHO mode for the base stations 14 on the activeset list or a subset thereof, and establishes SHO procedure with theappropriate base stations 14 (step 226). The scheduler at the basestation controller 10 will assign time slots for the SHO mode asdescribed above, and will send data packets to the base stations 14 onthe active set list or a subset thereof (step 228). The base stations onthe active set list will perform the joint space-time coding andtransmit data at slots assigned by the scheduler of the base stationcontroller 10 (step 230).

Next, the mobile terminal 16 will combine and decode the signals fromthe base stations 14 on the active set list, and will attempt to decodethe transmitted data (step 232). The mobile terminal 16 will thenattempt to decode the data received from the base stations 14 on theactive set list (step 234). If the data is properly decoded (step 236),the mobile terminal 16 will send an acknowledgement (ACK) to the basestations 14 on the active set list (step 238).

If the data is not properly decoded (step 236), the mobile terminal 16will send a negative-acknowledgement (NACK) to the base stations 14 onthe active set list (step 240). In response, the base stations 14 on theactive set list will perform joint space-time coding and re-transmit thedata (step 242). The mobile terminal 16 may then perform an automaticrepeat request (ARQ) or hybrid ARQ (HARQ) soft combining (step 244), andthe process will repeat.

During the transition to a SHO mode, the servicing base station 14 willhave data that needs to be transmitted and will not be able to bescheduled for multicast or non-multicast transmission by the basestation controller 10. Accordingly, the servicing base station 14 musttransmit the residual data to the mobile terminal 16 prior to fullyentering the SHO mode. In one embodiment, a single-cast technique isused where the servicing base station 14 transmits the residual data tothe mobile terminal 16 and the other base stations 14 on the active setlist do not transmit information in the channels or bands used by theservicing base station 14. Additional information on single-casting isprovided in greater detail later in this specification. Referring againto FIG. 8B, during transition to a SHO mode, the servicing base station14 will single-cast data to the mobile terminal 16 wherein the otherbase stations on the active set list will not transmit (step 246).Further, throughout the process of scheduling data for SHO mode mobileterminals 16, each base station 14 will autonomously schedule data fornon-SHO mode mobile terminals 16 (step 248).

With reference to FIG. 8C, throughout the process, the mobile terminal16 will continue to measure the pilot signal strength of all theadjacent base stations 14 (step 250) and calculate CIRs. Accordingly,the mobile terminal 16 may compute the CIR using equation 2 (step 252),and determine if the resultant CIR is greater than the threshold CIR Th₀(step 254). If CIR₁ is greater than threshold CIR Th₀ (step 254), themobile terminal 16 will update and report the active set list to theservicing base station 14 (step 256). Further, the base stationcontroller 10 will remove base station BS2 from the SHO mode for themobile terminal 16 (step 258). The base station BS2 is removed becausethe CIR of the servicing base station 14 is sufficient without use ofbase station BS2. Accordingly, the process will continue with step 226of FIG. 8B.

If the value of CIR₁ was not greater than threshold CIR Th₀ (step 254),the mobile terminal 16 will compute CIR using equation 3 (step 260). Ifthe value of CIR₂ is greater than threshold CIR Th₀ (step 262), thetwo-way SHO mode is still necessary, and the process will continue atstep 226 of FIG. 8B. If the value of CIR₂ is not greater than thethreshold CIR Th₀ (step 262), the mobile terminal 16 will compute thevalue of the CIR using equation 4 (step 264). Accordingly, if the valueof CIR₃ is not greater than threshold CIR Th₀ (step 266), the mobileterminal 16 will compute the value of CIR₄ (step 272), and so on and soforth until a sufficient number of base stations 14 are added to theactive set list to cause the value of CIR to exceed the threshold CIRTh₀.

If the value of CIR₃ is greater than the threshold CIR Th₀ (step 266),the mobile terminal 16 will update the active set list to include thebase station BS3 associated with I₃ and report the updated active listto the service base station 14 (step 268). At this point, the basestation controller 10 will add the base station BS3 to the SHO mode(step 270), and the process will continue at step 226 of FIG. 8B.

The data is transmitted from the base stations 14 to the mobileterminals 16 using unique space-time coding schemes. The followingoutlines two space-time-coding schemes involving transmission divisionin the frequency domain at each base station 14. For each scheme, twoembodiments are described. FIGS. 9 and 10 illustrate a MIMO-OFDM schemefor a mobile terminal 16 in a SHO-mode involving three base stations 14(BS 1 , BS 2 , and BS 3 ). Transmission division in the frequency domainis implemented in combination with space-time coding at each basestation 14. Such transmission division involves segregating theavailable OFDM frequency sub-bands among the participating base stations14. Each base station 14 only modulates the data it has been scheduledto transmit on the corresponding sub-bands. FIG. 10 illustrates thesub-band mapping among the three base stations 14 (BS 1 , BS 2 , and BS3 ) for one path of a dual path implementing space-time coding for agiven period of time. The other path will use the same sub-bands, butimplement different coding. The mapping control signal (FIG. 4) is usedto control mapping of the sub-bands. The base stations 14 arecoordinated via the base station controller 10 to select differentsub-bands for mapping control and STC encoding, as described herein, andto control power boosting.

For the first base station 14 (BS 1 ), the bottom third of the sub-bandsare used to modulate and transmit traffic data wherein the remainingtwo-thirds of the sub-bands are unused. Notably, the pilot signal isscattered throughout the traffic data, but not throughout the unusedsub-bands. For the second base station 14 (BS 2 ), the middle third ofthe sub-bands are used to modulate and transmit traffic data wherein theremaining two-thirds of the sub-bands are unused. For the third basestation 14 (BS 3 ), the top third of the sub-bands are used to modulateand transmit traffic data wherein the remaining two-thirds of thesub-bands are unused. For optimal performance, the power is boosted forthe active sub-bands to realize the full power transmission and cut forthe unused bands. Accordingly, the mobile terminal 16 will effectivelyreceive a different third of the frequency bands from each of the basestations 14 (BS 1 , BS 2 , and BS 3 ) and recover the corresponding databased on the STC and scheduling parameters. In some cases, the averagepower for the entire band remains within defined limits.

For non-multicast scheduling, different sub-packets are sent to eachbase station 14 (BS 1 , BS 2 , and BS 3 ), which will organize the datato effect the frequency division mapping and provide the space-timecoding for two antennas as described above. Accordingly, each basestation 14 is transmitting unique data. Each active sub-band is powerboosted by 10 log₁₀ (x) dB, where x is the number of base stations 14 inSHO mode and is equal to three in this example. The mobile terminal 16receives the entire frequency band, a portion from each base station 14,and performs space-time decoding to retrieve the packet data.

For non-multicast scheduling, the same packets are sent to each basestation 14 (BS1, BS2, and BS3), which will organize the data to effectthe frequency division mapping and provide the space-time coding for twoantennas as described above. Accordingly, each base station 14 istransmitting the same data at the same time, albeit in differentformats. Again, each active sub-band is power boosted by 10 log₁₀ (x)dB. The mobile terminal 16 receives the entire frequency band, a portionfrom each base station 14, and performs space-time decoding anddiversity combing to retrieve the packet data. Both of the above optionscan achieve SHO gain, which provides CIR improvement, by converting thetransmission power of a SHO base station 14 from interference into auseful signal. The first option allows high data throughput, but withoutmacro-diversity combining gain, wherein the second option yields a lowerthroughput, but provides macro-diversity gain. In general, the number ofparticipating base stations 14 in SHO made can be reduced with thesecond option. Notably, there are several possible designs for thesub-band division, which may include interlacing and the like. Based onthe teachings herein, those skilled in the art will recognize thevarious combinations to segregate the sub-bands among the participatingbase stations 14.

FIGS. 11 and 12 depict another MIMO-OFDM SHO scheme with joint basestation diversity. In this embodiment, each base station 14 (BS1BS1,BS2, and BS3) is associated with two antennas 28 (α and β). Unique tothis embodiment is that spatial diversity is provided across basestations 14. As illustrated, the STC encoding results in two STC datastreams, which are respectively transmitted from antennas at differentbase stations 14.

For non-multicast scheduling, a packet is divided into three uniquesub-packets and sent to the base stations 14 (BS1, BS2, and BS3),respectively. Base station 14 (BSI) antenna α and Base station 14 (BS2)antenna β perform the space-time encoding for the first sub-packet; basestation 14 (BS2) antenna a and base station 14 (BS3) antenna α performthe space-time encoding for the second sub-packet; and base station 14(BS3) antenna β and base station 14 (BS 1) antenna β perform thespace-time encoding for the third sub-packet. Each antenna pairtransmits one sub-packet, which is mapped onto one-third of the OFDMtime-frequency sub-bands. The remaining two-thirds of the sub-bands areempty and not used for data transmission. Each transmitted sub-band ispower boosted by 10 log₁₀ (x)dB, where x is the number of base stations14 in SHO mode and is equal to three in this example. The mobileterminal 16 receives the entire frequency band and performs space-timedecoding to retrieve the packet data.

For non-multicast scheduling, each packet is redundantly sent to thethree base stations 14 (BS1, BS2, and BS3). Base station 14 (BS1)antenna α and Base station 14 (BS2) antenna β perform the space-timeencoding for the packet; base station 14 (BS2) antenna α and basestation 14 (BS3) antenna α perform the space-time encoding for thepacket; and base station 14 (BS3) antenna β and base station 14 (BS1)antenna β perform the space-time encoding for the packet. Each antennapair transmits a copy of the packet, which is mapped onto one-third ofthe OFDM time-frequency sub-bands. The remaining two-thirds of thesub-bands are empty and not used for data transmission. Each transmittedsub-band is power boosted by 10 log₁₀ (x) dB. Again, x is the number ofbase stations 14 in SHO mode and is equal to three in this example. Themobile terminal 16 receives the entire frequency band and performsspace-time decoding to retrieve the packet data.

The joint STC scheme of FIG. 11 provides additional space-time codinggain over that provided in FIG. 9. The above examples for the MIMO-OFDMSHO space-time coding arrangement can be easily generalized into 2-way,3-way and N-way SHO operation. Because of the frequency divisionproperty of OFDM systems, part of the band can be used for SHO while theremainder of the band is used for transmitting the data packet tonon-SHO users by each base station 14. This provides more flexibility tothe scheduling for multi-users applications.

During the transition from a non-SHO mode to a SHO mode, the basestations 14 will have residual data, which needs to be transmitted tothe mobile terminals 16 and cannot be scheduled at the base stationcontroller 10. Accordingly, some embodiments use a single-castingtechnique, wherein data delivery may be orchestrated such that only onebase station 14 transmits data during SHO mode on select sub-bands whilethe other participating base stations 14 avoid using the sub-bands usedby the base station 14 to send the data. In this manner, interferenceassociated with the sub-bands of the other base stations 14 isminimized. During single-casting, joint scheduling and processingassociated with combing data received in part or whole from multiplebase stations 14 is unnecessary, since the entire data is sent from onlyone base station 14. Again, boosting power for the active sub-carriersis beneficial. Once the residual data has been transmitted to the mobileterminals 16, the multicasting or non-multicasting for mobile terminals16 operating in a SHO mode takes over, wherein the base stationcontroller 10 schedules data, which is either multicast ornon-multicast, to the base stations 14 on the active set list, and thentransmitted to the mobile terminals 16.

As noted above, an important element for STC decoding is accuratechannel estimation. The scattered pilot patterns are designed for theadjacent base station's pilot signal re-use planning. A scattered pilotpattern can have cyclic layout on the time-frequency plane. In order toachieve high quality channel estimation for the space-time decoding, theinterference from the adjacent base stations 14 must be minimized. In atleast one embodiment, power may be boosted for each base station'sscattered pilot singles, while for the same sub-carrier location of theall the other base stations 14, these sub-carrier transmissions shouldbe turned off to create a power null as illustrated in FIG. 13. Withthis arrangement, the scattered pilot sub-carriers are almost free fromthe co-channel interference.

Because the distances between mobile terminals 16 and base stations 14are different for each set, there is a relative transmission delaybetween the signals from the different base stations 14. During the basestation identification and timing synchronization stage, the mobileterminal 16 has already measured the timing synchronization positionscorresponding to different SHO base stations 14 in the active set list.In the SHO mode, the earliest arrival time from a particular basestation 14 is used as the synchronization position. As a result, onlyone base station 14 can be in perfect timing synchronization, while theothers have certain time offsets.

In general, an OFDM signal can tolerate time offsets up to thedifference of the prefix and the maximum channel delay. As long as thetime offset is within this tolerance, the orthogonality of thesub-channel is preserved. However the time offset will cause anadditional phase rotation, which increases linearly with respect to thesub-channel index. For non-coherent detection, no channel information isneeded, so the same STC decoding method as used in the non-SHO mode canbe applied by mobile terminal 16, if the differential encoding directionis performed along time. However, for coherent detection, accuratechannel information is necessary. The time offset may cause problemsduring channel reconstruction.

Let X, Y, H represent the transmitted signal, received signal and thechannel response in a frequency domain, respectively and ignore noise.For a 2×2 case (two transmit and receive paths):Y(k)=H(k)X(k)where

${{Y(k)} = \begin{bmatrix}{Y^{1}(k)} \\{Y^{2}(k)}\end{bmatrix}},{{X(k)} = \begin{bmatrix}{X^{1}(k)} \\{X^{2}(k)}\end{bmatrix}},{{H(k)} = \begin{bmatrix}{h_{11}(k)} & {h_{21}(k)} \\{h_{12}(k)} & {h_{22}(k)}\end{bmatrix}},$and k is the sub-carriers index.

-   If there is a time offset, the above relation should be modified as    Y(k)=H′(k)X(k)    where:

${{H^{\prime}(k)} = \begin{bmatrix}{h_{11}^{\prime}(k)} & {h_{21}^{\prime}(k)} \\{h_{12}^{\prime}(k)} & {h_{22}^{\prime}(k)}\end{bmatrix}},{{h_{ij}^{\prime}(k)} = {{h_{ij}(k)}{\varphi_{i}(k)}}},{{\varphi_{i}(k)} = {\exp\left( {{- {\mathbb{i}}}\; 2\;\pi\; k\;\delta\;{t^{i}/N_{FFT}}} \right)}},$φ_(i) is the additional phase rotation introduced by the time offset fori^(th) transmit antenna, δt^((i)) is the time offset in samples causedby the timing synchronization error for the signals from i^(th) transmitantenna. δt^((i)) is known during base station identification and timingsynchronization.

Theoretically the equivalent channel response H′ can be estimated andcompensated with the help of pilot signals. However, since the channelestimation is based on the scattered pilots, care must be taken tocompensate for relative transmission delay. The design principle of thedensity of the scattered pilots is to allow the reconstruction of thetime and frequency varying channel response. The spacing between pilotsin time direction is determined by the expected maximum Dopplerfrequency, while the spacing between pilots in the frequency directionis determined by the expected delay spread of the multi-path fadingchannel. The grid density of the scattered pilot pattern can provideenough sampling for the reconstruction of the propagation channelthrough interpolation. On the other hand, 9 varies with the sub-carrierindex, and its variation frequency increases with the increment of timeoffset. Therefore, the correlation bandwidth of the total equivalentchannel response H′ is determined by both the multi-path fading channeland the uncorrected time offset. As mentioned above, there is a timeoffset for the signals from the more distant base stations 14 because ofthe existence of the relative transmission delay. For example, in a 2×2MIMO-OFDM system, 4 channels are needed for channel estimation. Two ofthem may have relatively large time offsets, and as a result, a fastadditional phase rotation φ. Notably, the time offset will introducefast phase rotation. When the variation of φ is much faster than that ofH′, the grid density of the scattered pilots may not satisfy thesampling theorem; therefore, H′ cannot be obtained correctly byinterpolation.

To obtain correct channel information for all the multiple channelsduring SHO, a compensation method can be applied. The idea is that onlythe propagation channel is interpolated, for the variation of φ isknown. After FFT processing, the received time domain samples aretransferred to frequency domain components. Then, h_(ij)′(k) can beobtained at pilot sub-carriers k. Before interpolation is used to obtainthe channel response for all the sub-carriers, the contribution from φcan be removed by multiplying h_(ij)′(k) with the conjugate of φ_(i)(k),{tilde over (h)}ij(k)=h _(ij)′(k)φ_(i)*(k)It should be noted that only the channels related to the base station 14with time offset should be compensated. After interpolation, the channelresponse, {tilde over (h)}_(ij) of all the useful sub-carriers areobtained. The total equivalent channel responses h_(ij)′ of all theuseful sub-carriers are obtained by multiplying {tilde over (h)}_(ij)with φ_(i).

In essence, the channel responses for each of the data sub-carriers ofthe OFDM signal are compensated for transmission delays associated withtransmission from each of the multiple base stations 14 participating inthe OFDM soft handoff. In general, the mobile terminal 16 will use theunique PN codes provided in the preambles of each of the pilot signalsfrom each of the base stations 14 to determine the relative transmissiondelays from each of the base stations 14 participating in the OFDM softhand-off. After a fast Fourier transform, the scattered pilot signals ofthe OFDM signals are extracted in the frequency domain for each receiversection. Channel responses for the scattered pilot signals are estimatedfor each transmit channel. Any additional phase rotation caused by thetransmission delays from the estimated channel responses are removed, insome cases, using the multiplication techniques described above. At thispoint, the channel responses for the scattered pilot signals are known,and are used to interpolate the channel responses for the datasub-carriers in the OFDM signal. Once the channel responses for the OFDMdata sub-carriers are estimated, the phase rotation caused by thetransmission delays are added to the channel responses for each of theOFDM sub-carriers to provide the actual channel estimates to use duringreceiving transmissions from the various base stations 14.

Some embodiments provide an efficient soft handoff technique for OFDMsystems and improve data rates while minimizing interference associatedwith OFDM communications with mobile terminals at cell borders. Thoseskilled in the art will recognize improvements and modifications to thevarious embodiments described. All such improvements and modificationsare considered within the scope of the concepts disclosed herein and theclaims that follow.

What is claimed is:
 1. A mobile terminal comprising: receive circuitryadapted to receive a plurality of orthogonal frequency divisionmultiplexing (OFDM) signals; transform logic configured to provide atransform on each of the plurality of OFDM signals to generate aplurality of spatially coded signals; decoder logic configured toprovide spatial decoding on the plurality of spatially coded signals torecover scheduled data from servicing base stations; and processinglogic configured to: monitor signal strength from a plurality of basestations; identify ones of the plurality of base stations having signalstrength over a defined threshold; and enter a mode in which the mobileterminal is enabled to simultaneously communicate with one or more basestations.
 2. The mobile terminal of claim 1 further comprising: transmitcircuitry configured to transmit information identifying the ones of theplurality of base stations having signal strength over the definedthreshold.
 3. The mobile terminal of claim 1, wherein the mode in whichthe mobile terminal is enabled to simultaneously communicate with theone or more base stations further comprises an ability to:simultaneously communicate with multiple base stations; and receive andprocess different data packets from two or more base stations of themultiple base stations.
 4. The mobile terminal of claim 3, wherein thedifferent data packets comprise unique information from each of the twoor more base stations.
 5. The mobile terminal of claim 4 furtherconfigured to communicate with each of the two or more base stations ondifferent transmission channels.
 6. The mobile terminal of claim 5,wherein the processing logic is further configured to: determine achannel estimate for each of the different transmission channels; andprovide a respective channel response for each of the differenttransmission channels effective to recover the unique information fromeach of the two or more base stations.
 7. The mobile terminal of claim6, wherein the processing logic is further configured to recover theunique information from each of the two or more base stations byremoving transmission channel effects from each different transmissionchannel based, at least on part, on the respective channel response. 8.The mobile terminal of claim 1, wherein the one or more base stationscomprises a plurality of base stations.
 9. One or morecomputer-readable, hardware storage memories embodying one or moreprocessor-executable instructions which, responsive to execution by atleast one processor, enable a device to: receive a plurality oforthogonal frequency division multiplexing (OFDM) signals; provide atransform on each of the plurality of OFDM signals to generate aplurality of spatially coded signals; provide spatial decoding on theplurality of spatially coded signals to recover scheduled data fromservicing base stations; monitor signal strength from a plurality ofbase stations; identify ones of the plurality of base stations havingsignal strength over a defined threshold; and enter a mode in which thedevice is enabled to simultaneously communicate with one or more basestations.
 10. The one or more computer-readable, hardware storagememories of claim 9, the processor-executable instructions furtherconfigured to enable the device to transmit information identifying theones of the plurality of base stations having signal strength over thedefined threshold.
 11. The one or more computer-readable, hardwarestorage memories of claim 9, the one or more processor-executableinstructions are further configured to enable the device to:simultaneously communicate with multiple base stations; and receive andprocess different data packets from two or more base stations of themultiple base stations.
 12. The one or more computer-readable, hardwarestorage memories of claim 11, wherein the different data packetscomprise unique information from each of the two or more base stations.13. The one or more computer-readable, hardware storage memories ofclaim 12, wherein the one or more processor-executable instructions arefurther configured to enable the device to communicate with each of thetwo or more base stations on different transmission channels.
 14. Theone or more computer-readable, hardware storage memories of claim 13,wherein the one or more processor-executable instructions are furtherconfigured to enable the device to: determine a channel estimate foreach of the different transmission channels; and provide a respectivechannel response for each of the different transmission channelseffective to recover the unique information from each of the two or morebase stations.
 15. The one or more computer-readable, hardware storagememories of claim 14, wherein the one or more processor-executableinstructions are further configured to enable the device to recover theunique information from each of the two or more base stations byremoving transmission channel effects from each different transmissionchannel based, at least on part, on the respective channel response. 16.A computer-implemented method comprising: receiving a plurality oforthogonal frequency division multiplexing (OFDM) signals; providing atransform on each of the plurality of OFDM signals to generate aplurality of spatially coded signals; providing spatial decoding on theplurality of spatially coded signals to recover scheduled data fromservicing base stations; monitoring signal strength from a plurality ofbase stations; identifying ones of the plurality of base stations havingsignal strength over a defined threshold; and entering a mode associatedwith simultaneously communicating with one or more base stations. 17.The computer-implemented method of claim 16, the method furthercomprising: simultaneously communicating with multiple base stations;and receiving and processing different data packets from two or morebase stations of the multiple base stations.
 18. Thecomputer-implemented method of claim 17, wherein the different datapackets comprise unique information from each of the two or more basestations.
 19. The computer-implemented method of claim 18 furthercomprising communicating with each of the two or more base stations ondifferent transmission channels.
 20. The computer-implemented method ofclaim 19 further comprising: determining a channel estimate for each ofthe different transmission channels; and providing a respective channelresponse for each of the different transmission channels effective torecover the unique information from each of the two or more basestations.